In a manufacturing process for a thin-film transistor, abbreviated to TFT, as used in an active matrix liquid crystal display device, a batch light exposure system in which a TFT substrate is exposed to light at a time using a mirror projector type light exposure device, referred to herein as an aligner, and a multi-domain light exposure system, in which a TFT substrate is divided into plural domains for exposure to light using a lens type scan light exposure apparatus, referred to herein as a stepper, are nowadays in use.
Recently, there is an increasing demand for a larger size of a manufacturing apparatus for a liquid crystal display device in order to keep pace with the increasing size of the screen of the liquid crystal display device. As for a light exposure apparatus for a photoresist, an image frame is divided into plural domains, using a stepper, to effect light exposure from one domain to the next, whereby it is possible to cope with the increasing size of the image frame, even with a conventional light exposure apparatus. With this method, a light exposure shot system (FIG. 8) is used by a method shown in FIG. 7, in order that the volume of light exposure in the overlapping portions of junction areas of the plural domains will be uniform.
With this method, the optical system which exposes an end part of a reticle is provided with a blind 732 carrying a filter 731 presenting a gradient in transmittivity for smoothly changing the amount of light exposure at the junction of the boundary portions between shots of light exposure carried out in accordance with the multi-domain light exposure system. This produces moderate changes at the ends of the reticle, as indicated in a curve in a lower part of FIG. 8. The boundary portions are exposed to light in duplication to assure a uniform total light exposure volume.
However, the known system suffers from a drawback that the line of division is perceived at the time of display of the liquid crystal display device due to overlap deviation at the junction areas of the division or variations in illuminance of the exposure light. As a method for moderating the perceptibility of the division line, there is disclosed in Cited Reference 1 a method for complementary light exposure by plural neighboring masks.
This method is now explained with reference to FIG. 9. FIG. 9a shows a shot A 941 and a shot B 942 neighboring to each other in the transverse direction and an area of superposition 943 of these two shots A 941 and B 942. Referring to FIG. 9b, showing the area of superposition 943 to an enlarged scale, the shots are arranged in a random number fashion so that a larger number of the shots are distributed in an area proximate to the area of specific shots, in such a manner that the difference in luminance is not perceived by not providing a boundary line.
[Cited Reference 1]
Publication of JP Patent Kokai Publication JP-P2000-180894A (paragraph number [0016], FIGS. 9a and 9b).